A
response from Valve to a thread about offline mode on Steam indicates that
the way offline mode currently requires users to go online periodically to allow
them to play their Steam games without being connected to the Internet is a bug,
and not working as intended. He does not offer a timeframe for when this might
be fixed, but clearly states the requirement to go online once your game is
authenticated is a "bug" (thanks Creston). Here's word:

This is not
actually true - Offline Mode is designed to be indefinite. You can't access any
of Steam's online features such as friends lists or saved game synchronization,
of course, but the client should allow you to run in Offline Mode for as long as
you like.

That said, there are many components involved in Offline Mode, and some of them
have known issues and bugs which we are continually working to improve. We're
aware that it doesn't always work as flawlessly as we want it to, but please
keep reporting bugs with Offline Mode. It is not broken 'by design'.

Post CommentEnter the details of the comment
you'd like to post in the boxes below and click the button at
the bottom of the form.